Chemical vapor deposition of fluorine-doped zinc oxide

ABSTRACT

Fims of fluorine-doped zinc oxide are deposited from vaporized precursor compounds comprising a chelate of a dialkylzinc, such as an amine chelate, an oxygen source, and a fluorine source. The coatings are highly electrically conductive, transparent to visible light, reflective to infrared radiation, absorbing to ultraviolet light, and free of carbon impurity.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

The invention was made under a contract with the Department of Energy ofthe United States Government, DOE Grant Number XAN-4-13318-05.

This application is a national stage of PCT/US97/11552 filed Aug. 13,1997.

BACKGROUND OF THE INVENTION

Zinc oxide is a wide-bandgap semiconductor that is transparent tovisible light. U.S. Pat. No. 4,751,149 to Vijayakumar et al. disclosedthat the electrical conductivity of zinc oxide can be increased greatlyby adding small amounts of the group 13 elements boron, aluminum,gallium or indium. Addition of these dopants, however, decreases thetransparency of the zinc oxide. U.S. Pat. No. 4,990,286 to Gordondisclosed that fluorine is also effective in increasing the electricalconductivity of zinc oxide films, and that the resulting films havehigher transparency than those doped with group 13 dopants. The greatertransparency of the fluorine-doped films gives them better performancein devices such as solar cells, flat-panel display devices,electrochromic absorbers and reflectors, energy-conserving heat mirrors,and anti-static coatings.

Zinc oxide may be formed by reactions of vapors of a wide variety ofzinc-containing compounds. These processes are called chemical vapordeposition (CVD) processes.

Some widely-used zinc CVD precursors are dialkylzinc compounds, havingthe general form ZnR₂, in which R stands for an organic radical, such asmethyl, ethyl, isopropyl, etc. Examples of these precursors aredimethylzinc and diethylzinc. In U.S. Pat. No. 4,751,149 vapors of adialkylzinc compound are mixed with water vapor and diborane near aheated surface, on which a layer of boron-doped zinc oxide is deposited.In U.S. Pat. No. 4,990,286 vapors of a dialkylzinc compound are mixedwith alcohol vapor and hexafluoropropene near a heated surface, on whicha layer of fluorine-doped zinc oxide is deposited.

Dialkylzinc compounds have some disadvantages in a CVD process.Dialkylzinc compounds are pyrophoric, that is they ignite spontaneouslyin air, so that they are a serious fire hazard. Even small amounts ofoxygen in the CVD chamber cause powdered zinc oxide to form. Thus theprior art CVD processes for forming zinc oxide films from dialkylzinccompounds can be disrupted by air leaks into the CVD chamber.

Complexes of dialkylzinc compounds with tetrahydrofuran have beenproposed as CVD sources for zinc oxide by T. Kaufmann, G. Fuchs and M.Webert, in Crystal Research & Technology, vol. 23, pp. 635-639 (1988).However, these complexes are not very stable, and they dissociate in thegas phase unless a more than 100-fold excess of tetrahydrofuran is used.

Non-pyrophoric zinc CVD sources have also been found. Zinc propionatewas discovered to be a CVD source material for zinc oxide films by Y. A.Savitskaya and L. A. Ryobova, Zhurnal Prikladnoi Khimii, Vol. 37, pp.796-800 (1962). Zinc acetylacetonate was used similarly by L. A.Ryabova, Y. S. Savitskaya and R. N. Sheftal, Izvestia Akademii Nauk.SSSR, Neorganischeskie Materialy, Vol. 4, p. 602 (1968). Zinc acetatewas claimed as a CVD precursor for films with electronic purposes by M.Hattori and T. Maeda in Japan. Kokai 73 29,699 (1973). Thesenon-pyrophoric sources are solids, so they are less convenient to handlethan liquids are, and they are more difficult to vaporize.

SUMMARY OF THE INVENTION

A principal object of the present invention is to deposit fluorine-dopedzinc oxide having high electrical conductivity, high visibletransparency, high infrared reflectivity, and high purity.

Another object of the present invention is the deposition offluorine-doped zinc oxide films from precursors that are not pyrophoric.

A further object of the present invention is to deposit fluorine-dopedzinc oxide from a robust chemical vapor deposition process which is noteasily disrupted by leaks of air into the deposition region.

A related object is to deposit fluorine-doped zinc oxide by a chemicalvapor deposition process in which all the reactant vapors may be mixedhomogeneously before delivery to the heated surface of the substrate.

Another object of the invention is to provide a chemical vapordeposition process for fluorine-doped zinc oxide from reactants that arecommercially available, stable and relatively non-hazardous.

Other objects of the invention will be obvious to those skilled in theart on reading the instant invention.

The above objects have been substantially achieved by a process for thechemical vapor deposition of fluorine-doped zinc oxide from vaporscomprising a chelate of a dialkylzinc compound, an oxygen source, and afluorine source. One preferred embodiment uses a vapor mixture of theN,N,N',N'-tetraethylethylenediamine chelate of diethylzinc, ethanol,hexafluoropropene and optionally, an inert carrier gas such as nitrogen.This vapor mixture is brought into contact with a substrate heated to atemperature in the range of about 400 to 500° C., most preferably about425 to 475° C. Under these conditions, a transparent, electricallyconductive, infrared reflective film of fluorine-doped zinc oxide isdeposited on the substrate. The N,N,N',N'-tetraethylethylenediaminechelate of diethylzinc is a non-pyrophoric liquid, in contrast to thehazardous, pyrophoric liquid diethylzinc.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention can be carried out in standard equipmentwell known in the art for chemical vapor deposition. The apparatusbrings the vapors of the reactants into contact with a heated substrateon which the metal oxide deposits. The process can operate at a varietyof pressures, including in particular normal atmospheric pressure, andalso lower pressures.

The reactants may be vaporized by means well-known in the art, includingpassing a carrier gas through a heated liquid or powdered solid in abubbler, or injecting or spraying a liquid or a solution into a flow ofheated carrier gas.

The zinc chelates suitable for the practice of this invention consist ofdialkylzinc molecules each of which has its zinc atom bonded to two ormore nitrogen atoms in a tertiary organic diamine or polyamine. Althoughit is expected that oxygen, sulfur and phosphine based organic chelateswill have a lower affinity for dialkylzinc, such chelates are alsocontemplated as within the scope of the invention. Zinc chelates withorganic amines provide a non-pyrophoric liquid or soluble solid, whichcan be easily vaporized for use in CVD processes. Further, thesecompounds have been demonstrated to be reactive CVD source materials forpreparation of zinc oxide films.

Many of the chelates were found to be liquids at room temperature. Thosethat were solids at room temperature were found to be very soluble inxylene. The xylene solutions are convenient for transferring andvaporizing these compounds.

The N,N,N',N'-tetraethylethylenediamine chelate of diethylzinc may beprepared by slowly adding diethylzinc to dryN,N,N',N'-tetraethylethylenediamine while stirring under an inertnitrogen atmosphere and cooling to remove the heat of reaction. Theproduct is a non-pyrophoric liquid that can be distilled at 92° C. undera pressure of 20 millibar.

The other chelates used in the following examples were prepared in asimilar way. The required diamines are all commercially available,except for the N-ethyl-N',N',N'-trimethylethylenediamine used in Example4. The N-ethyl-N',N',N'-trimethylethylenediamine was prepared fromcommercially available N,N,N-trimethylethylenediamine by reacting itwith acetic anhydride, and neutralizing the product with NaOH. Then theresulting amide was reduced with lithium aluminum hydride in ethersolution. After hydrolysis and filtration, the product was extracted inether and separated by distillation.

Any oxygen-containing compound which exhibits affinity for zinc andwhich demonstrates an ability to deposit oxygen on heated substratesurfaces may be used in the deposition process. Alcohols are preferredsources for the formation of zinc oxide film, although otheroxygen-containing sources such as ethers, may be used.

Zinc oxide films containing some carbon impurity can make the filmabsorb visible light, giving it a brown color. Adding an excess ofalcohol vapor to the gas mixture substantially eliminates the carbonimpurity and the brown color, giving clear, transparent zinc oxidefilms. It is preferred to use at least 10 times as many moles of alcoholvapor as zinc chelate.

In practicing this deposition process, we observed that nucleation ofthe zinc oxide films on glass surfaces can be erratic and notreproducible. This effect was easily seen by observing samples fromdifferent runs, which showed different amounts of diffusely scatteredlight, called haze. This haze arises from light scattered by the tinycrystals of zinc oxide in the films. When zinc oxide nuclei form at ahigher density on a glass surface, the crystals are smaller and causeless haze. When the glass surface was first coated with a thin film ofaluminum oxide, subsequent deposition of zinc oxide showed a high andvery consistent nucleation density.

An explanation of the effect of aluminum oxide precoating can be basedon the well-known catalytic effect that aluminum oxide surfaces have onthe decomposition of alcohols into water and unsaturated hydrocarbons.The water produced by this decomposition rapidly reacts with the zincprecursors to nucleate and deposit zinc oxide on the surface of thealuminum oxide. In contrast, the undecomposed alcohol produces zincoxide much more slowly and nucleates less efficiently. The same effectwas shown by precoating the glass with other catalytically active oxidefilms, including titanium dioxide, tin dioxide and zinc oxide.

Aluminum oxide with small additions of titanium dioxide is aparticularly effective precoating for producing highly transparent zincoxide films, because the refractive index (n) of such a mixture can beadjusted to be intermediate (n=1.7) between that of the glass (n=1.5)and that of the fluorine-doped zinc oxide (n=1.9), thereby reducing theamount of light lost in reflection.

In the following examples, the glass substrates were first coated withamorphous aluminum oxide by chemical vapor deposition at 550° C. glasstemperature from a vapor mixture including aluminum acetylacetonate andoxygen, as described in example 2 of U.S. Pat. No. 4,187,336. Anadditional benefit of using the amorphous aluminum oxide is that itprevents diffusion of sodium from the glass substrates into thefluorine-doped zinc oxide film, whose electrical conductivity can bedecreased by sodium. Aluminum oxide is also highly resistant to etchingby fluorine-containing reaction byproducts, such as hydrogen fluoride,which might otherwise attack the glass surface.

When the deposition process is carried out at substrate temperaturesaround 400° C., the zinc oxide films were smooth and had low haze. Suchclear films are preferred for applications such as liquid crystaldisplays. When the substrate temperature during deposition was increasedto around 450° C., the crystallites of zinc oxide become larger, andfilms greater in thickness than about 0.5 micrometer scattered somelight, giving a hazy appearance. Such hazy films can be used to increasethe amount of light absorbed by solar photovoltaic cells.

Adding a small amount of fluorine (typically less than about one atomicper cent) to zinc oxide dramatically increases its electricalconductivity. In the practice of this invention, a suitably reactivefluorine-containing vapor is added to the zinc chelate vapor and theoxygen-containing vapor, to deposit fluorine-doped zinc oxide on aheated substrate. Particularly suitable fluorine sources arefluorocarbon compounds, which contain fluorine bound to carbon atoms.Fluoroalkenes have sufficient reactivity to introduce fluorine into thegrowing zinc oxide film, while fluoroalkanes are unreactive under theconditions used. Hexafluoropropene is one preferred fluoroalkene sourceof fluorine, and tetrafluoroethene is another. Other sufficientlyreactive fluorocarbon compounds are acetyl fluoride, carbonyl fluoride,benzoyl fluoride and hexafluoropropene oxide.

EXAMPLE 1

The liquid N,N,N',N'-tetraethylethylenediamine chelate of diethylzincwas vaporized by injecting it at a rate of 0.1 milliliters/min from asyringe pump into a flow of 4 liters per minute of dry nitrogenpreheated to a temperature of 200° C. Dry ethanol was placed in abubbler and heated to 70° C. Dry nitrogen flowed through the bubbler ata rate of 0.5 liter/minute, and the vapor mixture exiting from thebubbler was diluted by an additional flow of 0.8 liters/minute of drynitrogen. Hexafluoropropene gas flowed from a pressurized supply tank ata rate of 0.05 liters/minute into the ethanol/nitrogen gas mixture.

These two vapor mixtures were combined at a tee joint, from which theyflowed through a line heated to 200° C. into the entrance to a CVDchamber measuring 10 cm wide by 0.6 cm high by 7 cm long (in thedirection of the gas flow). A glass substrate rested on the bottom ofthe CVD chamber, which is heated from below, so that the glass plate washeld at a temperature of about 450° C., while the top plate of the CVDchamber was at about 300° C.

Prior to the deposition and while the glass plate was heating up, drynitrogen passed through the chamber. Valves then switched on the flowsof zinc chelate vapor and ethanol through the chamber for five minutes,during which a layer of fluorine-doped zinc oxide formed. Then thereactant flows were stopped, the nitrogen flow was returned to thechamber, and the glass plate was removed.

On the surface of the glass plate there was a transparent coating offluorine-doped zinc oxide with a maximum thickness of about 500nanometers. The sheet resistance was as low as 10 ohms per square, andthe absorption of visible light was not more than 2%. Analysis of thefilm showed that it consisted of nearly stoichiometric zinc monoxide andsmall amounts (less than 0.5 atomic percent) of fluorine.

EXAMPLE 2

Example 1 was repeated with the liquidN,N-diethyl-N',N'-dimethylethylenediamine chelate of diethylzinc inplace of the N,N,N',N'-tetraethylethylenediamine chelate of diethylzinc.A similar deposit of transparent, electrically conductive fluorine-dopedzinc oxide was formed.

EXAMPLE 3

Example 1 was repeated with the liquidN,N'-diethyl-N,N'-dimethylethylenediamine chelate of diethylzinc inplace of the N,N,N',N'-tetraethylethylenediamine chelate of diethylzinc.A similar deposit of transparent, electrically conductive fluorine-dopedzinc oxide was formed.

EXAMPLE 4

Example 1 was repeated with the liquidN-ethyl-N',N',N'-trimethylethylenediamine chelate of diethylzinc inplace of the N,N,N',N'-tetraethylethylenediamine chelate of diethylzinc.A similar deposit of transparent, electrically conductive fluorine-dopedzinc oxide was formed.

EXAMPLE 5

The N,N,N',N'-tetramethylethylenediamine chelate of diethylzinc is asolid at room temperature. It was found to be soluble to an extent ofmore than 50 weight per cent in the mixed isomers of xylene. Example 1was repeated with a 50% by weight solution in xylene of theN,N,N',N'-tetramethylethylenediamine chelate of diethylzinc, in place ofthe N,N,N',N'-tetraethylethylenediamine chelate of diethylzinc, exceptthat the pumping rate of the solution was increased to 0.2milliliters/minute. A similar deposit of transparent, electricallyconductive fluorine-doped zinc oxide was formed.

EXAMPLE 6

Example 5 was repeated with a 50% by weight solution in xylene of thelow-melting solid 1,4-dimethylpiperazine chelate of diethylzinc, inplace of the solution of N,N,N',N'-tetramethylethylenediamine chelate ofdiethylzinc. A similar deposit of transparent, electrically conductivefluorine-doped zinc oxide was formed.

EXAMPLE 7

Example 5 was repeated with a 50% by weight solution in xylene of thesolid N,N,N',N'-tetramethyl-1,3-propanediamine chelate of diethylzinc,in place of the solution of N,N,N',N'-tetramethylethylenediamine chelateof diethylzinc. A similar deposit of transparent, electricallyconductive fluorine-doped zinc oxide was formed.

EXAMPLE 8

Example 5 was repeated, except that the fluorine dopant was 0.5 weightper cent acetyl fluoride dissolved in the solution, and thehexafluoropropene fluorine gas was omitted. A similar deposit oftransparent, electrically conductive fluorine-doped zinc oxide wasformed.

The liquids and solutions used in these examples were all tested to benon-pyrophoric by the methods published by the United States Departmentof Transportation. The test involves dropping 0.5 milliliters of theliquid or solution on a Whatman No. 3 filter paper, and observing thatno flame or charring of the paper occurs.

EXAMPLE 9

Example 8 was repeated with 0.6 weight per cent of benzoyl fluoride inplace of the acetyl fluoride. A film with sheet resistance of about 10ohms per square was obtained.

EXAMPLE 10

The remaining unused solution from Example 9 was stored at roomtemperature for one day. Then another film of fluorine doped zinc oxidewas deposited using part of the same solution. The sheet resistance ofthe fluorine-doped zinc oxide film was reduced from about 10 ohms persquare to about 8 ohms per square.

Further samples were made with this same solution during the next month,and the resistances were all around 8 ohms per square. After about amonth, samples made from the old solution had higher resistances.

Those skilled in the art will recognize or be able to ascertain using nomore than routine experimentation, many equivalents to the specificembodiments of the invention described specifically herein. Suchequivalents are intended to be encompassed in the scope of the followingclaims.

What is claimed is:
 1. A process for forming transparent, electricallyconductive and infrared reflective fluorine-doped zinc oxide by reactionof a vapor mixture comprising a vapor of a chelate of a dialkylzinc, anoxygen-containing vapor, and a fluorine-containing vapor.
 2. The processof claim 1, wherein the chelate comprises an amine chelate.
 3. Theprocess of claim 1, in which the amine chelate of dialkylzinc is atertiary diamine chelate of a dialkylzinc.
 4. The process of claim 2, inwhich the tertiary diamine chelate of dialkylzinc is theN,N,N',N'-tetraethylethylenediamine chelate of diethylzinc.
 5. Theprocess of claim 2, in which the tertiary diamine chelate of dialkylzincis the N,N-diethyl-N',N'-dimethylethylenediamine chelate of diethylzinc.6. The process of claim 2, in which the tertiary diamine chelate ofdialkylzinc is the N,N'-diethyl-N,N'-dimethylethylenediamine chelate ofdiethylzinc.
 7. The process of claim 2, in which the tertiary diaminechelate of dialkylzinc is the N-ethyl-N',N',N'-trimethylethylenediaminechelate of diethylzinc.
 8. The process of claim 2, in which the tertiarydiamine chelate of dialkylzinc is theN,N,N',N'-tetramethylethylenediamine chelate of diethylzinc.
 9. Theprocess of claim 2, in which the tertiary diamine chelate of dialkylzincis the N,N,N',N'-tetramethyl-1,3-propanediamine chelate of diethylzinc.10. The process of claim 2, in which the tertiary diamine chelate ofdialkylzinc is the 1,4-dimethylpiperazine chelate of diethylzinc. 11.The process of claim 1, in which the reactive fluorine-containingcompound is an organic fluoride.
 12. The process of claim 11, in whichthe organic fluoride is selected from the group consisting ofhexafluoropropene, tetrafluoroethene, acetyl fluoride, carbonylfluoride, benzoyl fluoride and hexfluoropropene oxide.
 13. The processof any of the preceding claims, in which said vapor mixture contains analcohol.
 14. The process of claim 13, in which said alcohol is ethanol.15. The process of claim 1, in which said fluorine-doped zinc oxidelayer is formed on a layer comprising one or more oxides selected fromthe group consisting of aluminum oxide, titanium oxide, tin oxide andzinc oxide.
 16. The process of claim 1, in which said fluorine-dopedzinc oxide layer is formed on a layer of aluminum oxide.
 17. A processfor forming a layer comprising zinc oxide by reaction of a vapor mixturecomprising a vapor of an amine chelate of dialkylzinc and anoxygen-containing vapor.
 18. A process as in claim 17, in which theoxygen-containing vapor is an alcohol.
 19. A process as in claim 18, inwhich the alcohol is ethanol.
 20. The process of claim 17, wherein thezinc oxide layer comprises transparent, electrically conductive andinfrared reflective fluorine-doped zinc oxide.